(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method of forming a gate oxide layer.
(2) Description of the Prior Art
In the formation of Integrated Circuits on the surface of a semiconductor substrate, a layer of silicon oxide is typically grown over the surface of a monocrystalline substrate. The formation of this layer of stoichiometric and non-stoichiometric oxide is generally well known in the art. The layer of oxide can be grown either by means of deposition or by thermal oxidation techniques. Thermal oxidation is generally preferred for areas where electrical performance is critical. In the formation of for instance MOSFET devices, a polysilicon layer is deposited over the layer of oxide and patterned to form the polysilicon gate electrode of the MOSFET device.
The creation of high quality gate oxide is of critical importance in the fabrication of semiconductor devices; gate oxide quality has a direct effect on device yield, reliability and performance. Problems such as surface roughness, layer impurity and particles can be caused by cleaning technology parameters and aspects such as concentration of the cleaning solution, chemicals used during the cleaning process, the cleaning equipment, the cleaning recipe and others. Oxidation problems can be caused by oxidation process control parameters such as time, temperature, pressure, oxidation agents or gasses used and others. Silicon wafer technology can introduce problems of initial substrate defects and impurities. Atomic flatness needs to be achieved for the surface of the substrate in order to allow the deposition of a high quality oxide layer on its surface. Well known methods are available in the art to reduce substrate micro-roughness such as the choice of the cleaning temperature, the etchant to be used and the time that is required to establish the desired substrate surface quality.
For a number of applications, such as the creation of the thin layer of tunnel oxide for Electrically Erasable Programmable Read-Only Memory (EEPROM) devices, the oxide layer is grown over heavily doped regions. These oxide layers are typically thinner (less than 100 Angstrom thick) than gate oxides (in order to readily achieve programmability of the device) and are generally considered to be of lower quality than oxides grown over lightly doped regions. The channel region of MOS devices generally is created using lightly doped regions, the gate oxide created over this region therefore tends to be of relatively high quality.
Thermally grown oxides provide good adherence between the oxide layer and the underlying silicon substrate while also providing a good mask against ion implantation used for the formation of the gate electrode source/drain regions. Irregularities in thermally grown oxide may result from irregularities on the silicon surface over which the layer of oxide is grown. Such irregularities can take on many different forms and can be classified as precipitates, dislocations, defects, contaminants, improper bonds, etc. These surface irregularities may be compounded by irregularities that occur in the created layer of oxide. The accumulation of irregularities that are further amplified during the process of the creation of the oxide layer may lead to the formation of "trap sides" where charged ions or minority carrier may accumulate.
The gate oxide forms the interface between the underlying (doped) channel regions of MOS devices and the overlying gate electrodes of the MOS transistor. Increased emphasis on smaller device dimensions leads to thinner gate oxide layers. This however places further requirements on the quality of the gate oxide layer. In the MOS transistor, the gate dielectric must support the voltage difference between the gate electrode and the substrate in an environment where ions and electrons from the gate and substrate enter the gate electrode. For very small-scale MOS devices, the effective gate length can be less than 1 micron. U.S. Pat. No. 5,464,792 teaches that, at such small gate length, electrons can be injected into the dielectric layer during periods when the transistor is switched on and off. This injection can, over time, cause a shift in the threshold voltage of the dielectric, which may effect the switching ability of the device.
Decreasing the thickness of the oxide layer leads to increasing of the electro-magnetic field that is present inside and at the boundaries of the oxide layer. This leads to the creation of hot-carriers within this field that are injected into the gate dielectric. This leads to an increase of the density of charge carriers in the interface region resulting in a shift of the electrical characteristics of the device. This leads to serious problems of deterioration of operational margins of the device. These effects are to be avoided meaning that requirements must be placed on the fabrication of the gate dielectric that result in an electrically stable interface.
From the above it is clear that functionality and reliability problems can occur in advanced Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) that employ polysilicon or polycide (polysilicon/metal silicide stack) gate electrodes in contact with thin gate oxide layers. These problems are caused by the diffusion of dopant impurities and/or other impurities from the polysilicon or polycide gate electrode into and/or through the underlying thin gate oxide layer. It is, for example, known that boron and/or phosphorus dopant impurities employed in doping polysilicon can diffuse through an underlying thin gate oxide layer and diffuse into a device channel region. Fluorine impurities may also diffuse through or from the polysilicon layer of a polysilicon or polycide gate electrode and cause an apparent increase in an underlying thin gate electrode oxide layer thickness. Fluorine impurities may originate from either boron difluoride impurity doping of a polysilicon layer or from tungsten hexafluoride based CVD tungsten silicide employed in forming the upper layer of a gate electrode. This increase in thin gate oxide layer thickness may compromise the operating parameters of a MOSFET device.
From the above it is further clear the gate dielectric must not be prone to an excessive amount of charge build-up within the layer or at the dielectric to underlying substrate interface while the dielectric breakdown of the layer must be adequately high. The interface layer must exhibit good resistance to hot carrier penetration and subsequent damage while diffusion and penetration of the oxide layer by dopants and other charge carrying impurities is well understood and controlled.
U.S. Pat. No. 5,650,344 teaches that the use of re-oxidized nitride oxide material for gate oxides is a frequently applied practice. It has been shown that the use of re-oxidized nitrided gate oxides (ONO gates) provides a significant improvement in gate oxide quality with respect to gate degradation due to high field strength and radiation, further retarding boron diffusion from boron doped polysilicon gates and providing resistance against hot electrons in both p and n MOSFET devices. In addition, the high positive fixed charge at the edge of the polysilicon gates (that results from the use of ONO gates) increases the punch through voltage for p-type MOSFET.
U.S. Pat. No. 5,650,344 further teaches that these benefits have not all be derived from the use of the ONO gate but can also be attributed to the nitrogen region that forms in the substrate and in the gate oxide along their interface. The nitrogen region forms in both the substrate and in any overlying nitrided oxide material along the interface of the substrate and the overlying nitrided oxide, which may include all or part of the gate oxide. In Prior Art devices, this nitrogen region is uniform along the interface of the gate oxide and the underlying substrate and typically has nitrogen concentration levels of 10-20% by atomic weight.
U.S. Pat. No. 5,650,344 further teaches a disadvantage of the use of ONO gates and the accompanying underlying nitrogen regions. A uniform fixed-charge density region develops along the common interface and over the entire width of the gate oxide. The presence of the fixed-charge density regions under the center of the ONO gate increases electric charge scattering and thus degrades electric charge mobility between the gate oxide and the substrate.
Nitrogen concentrations must because of this effect be kept below about 4% whereby this level or slightly less of Nitrogen concentrations provides adequate protection against boron penetration from a polysilicon gate into the substrate. It is further known that a nitrogen concentration of about 10% or more is required under the periphery of the gate oxide to provide a high enough positive fixed charge density to sufficiently increase the punch-through voltage for p-type MOSFET.
Improved semiconductor devices may therefore be formed by exposing the gate electrode to a nitride environment after the polysilicon gate electrode has been formed on the surface of the substrate. The gate structure may serve as total or partial shield during this exposure, which may result in a reduction of the concentration of the re-oxidized nitrided material over the width of the gate electrode. The shielding by the gate electrode also reduces the concentration of nitride underlying the gate electrode.
FIGS. 1 through 3 shows Prior Art formation of nitrogen concentrations within an oxide layer.
FIG. 1 shows a cross section of a semiconductor substrate 10 over which a layer 12 of oxide has been deposited. The method includes oxidizing a silicon-based surface in an oxidizing atmosphere in a first temperature to form an oxidized silicon-based surface. The oxidized silicon-based surface is then heated in an inert atmosphere to a second temperature that is higher than the first temperature to anneal the oxidized silicon-based surface 14, FIG. 2. The inert atmosphere can contain NO or N.sub.2 O or NH.sub.3 and is any atmosphere that grows minimal oxide on or within a silicon-bases surface. While maintaining the second temperature, the inert atmosphere receives or is replaced with a nitrided oxide gas to form the nitrogen atmosphere. The nitride of the nitrided oxide gas penetrates the surface of the oxidized silicon-based surface and forms a concentration 16 of nitride that has accumulated at the interface between the annealed layer of oxide and the underlying substrate, FIG. 3. Thereafter, the oxidized silicon-based surface is cooled in the presence of both oxygen and the inert gas. The oxidizing atmosphere can contain, in addition with oxygen, also ozone, hydrogen, hydrogen peroxide, silicon dioxide or hydrogen and chlorine.
U.S. Pat. No. 5,851,892 (Lojek et al.) shows a post oxide N.sub.2 anneal.
U.S. Pat. No. 5,605,848 (Ngaoaram) shows a dual N I/I process for a gate oxide.
U.S. Pat. No. 5,403,786 (Hori) discloses an oxide growth, a nitriding process and an anneal for a gate oxide.
U.S. Pat. No. 5,672,521 (Barsan et al.) shows an N I/I into the substrate to grow gate oxides of different thicknesses.
U.S. Pat. No. 5,296,411 (Gardner et al.) discloses a N2 anneal for a gate oxide.
U.S. Pat. No. 5,650,344 (Ito et al.) recites a reoxidized nitrided oxide layer.
U.S. Pat. No. 5,464,792 (Tseng et al.) shows an anneal to create N buildup in an oxide layer.